1. Field of the Invention
The present invention generally relates to image forming apparatuses and methods for forming an image which form a color print image by printing a plurality of colors on one medium on which one color image is printed over other color images and, more particularly, to an image forming apparatus and a method for forming an image in which the accuracy of color placement on a medium is improved.
2. Description of the Related Art
Especially in an electronic color printer, a color image is formed by mixing toners of black and three primary colors: yellow, magenta and cyan.
There are methods for processing transcriptions of four color images as follows: a method using four photosensitive drums, in which one photosensitive drum is provided for each color and a transcription is processed for each color image on a sheet in sequence; a method using a single photosensitive drum, in which a toner image is developed on the photosensitive drum by selectively using one of four developing devices corresponding to one of four colors and a transcription is performed four times on a sheet; and a method using a single photosensitive drum and a transcription medium, in which each of four color toner images is developed on the transcription medium and the overlapped four color images are copied on a sheet at once. The methods have a disadvantage in that displacement between two color images occurs.
To eliminate deterioration of image quality caused by the displacement, each color has to be overlapped with an accuracy of a few .mu.ms. To achieve this accuracy, improvement of an accuracy of a photosensitive drum's rotation, improvement of an accuracy of transferring a sheet, or auto adjustment techniques by detecting color displacements have been discussed. However, it is still difficult to achieve enough accuracy to overlap colors and these improvements require a high cost.
Recently, it is desired to improve image quality and to reduce cost related to improving the color displacement.
In a conventional color image forming method, the following method is generally used.
For example, to produce a red color, two colors, yellow and magenta of the three primary colors, are overlapped on a sheet. However, when a red color image is formed, each of the yellow image and magenta image is processed in sequence but separately: that is, the yellow image and the magenta image are processed at slightly different times. Therefore, transcription of each color image is placed at a slightly different position on the sheet. When the displacement of color images is significant, the expected color is not produced and image quality is deteriorated.
Details of the above-mentioned displacement will be explained with reference to FIGS.1A and 1B.
FIG. 1A shows a diagram illustrating an example of a conventional color overlapped image in which there is no color displacement.
In this figure, M indicates magenta, R indicates red, and Y indicates yellow.
In a conventional method, only when a centerline of a magenta straight line is coincident with a centerline of a yellow straight line, will the color displacement not occur so that a clear red straight line is formed.
FIG. 1B shows a diagram illustrating an example of a color overlapped image which has displacement at a 100-.mu.m displacement allowance in a conventional manner.
It should be noted that the allowance displacement means a physical minimum amount at which human eyes begin to discern that two images are physically displaced. That is, the 100-.mu.m allowance displacement indicates that human eyes begin to discern a displacement between two images when the two images are displaced by at least 100 .mu.m. On the contrary, human eyes can not discern a displacement between two images when the two images are displaced by less than 100 .mu.m.
In FIG. 1B, a displacement 5 is a 100-.mu.m difference between magenta's centerline and yellow's centerline. In a conventional method, because the magenta line and the yellow line are of the same thickness, the displacement 5 causes a 100-.mu.m difference 6 between lower edges of the magenta straight line and the yellow straight line and also causes a 100-.mu.m difference 7 between upper edges of the magenta straight line and the yellow straight line.
Even with the discussed improvement of the accuracy of a photosensitive drum's rotation, the improvement of the accuracy of transferring a sheet and the auto adjustment techniques by detecting color displacements, it is still difficult to eliminate the color displacement when a certain color line is formed by overlapping two lines having the same thickness.